The invention relates to fiber optic chemical sensors (FOCS) and more particularly to reservoir FOCS.
The basic FOCS design is a tip coated FOCS in which a reagent that specifically and sensitively interacts with the analyte of interest is placed or attached at the tip of an optical fiber. Fibers in the 100 to 600 micron diameter range are most often used. Unfortunately, because of the very small surface areas at their tips (7.times.10.sup.-5 to 3.times.10.sup.-3 cm.sup.2) it is difficult to get enough chemistry at the end of the fiber for reliable measurements, i.e. good signal to noise.
Several approaches have been used to get enough chemistry onto the tip of the fiber and thus overcome the surface area limitations, including: (i) surface amplification techniques, a unique type of covalent immobilization in which the sensing material is attached to the tip by a surface amplification polymer; (ii) imbedding the chemistry into a membrane placed at the tip of the fiber; (iii) large surface area porous glass fibers attached to the tip; and (iv) use of very sensitive chemistry, i.e. fluorescence reactions. In special situations, where the species to be measured is volatile, reservoir FOCS (liquid reagents) with gas permeable membranes have been used. Each of these has shown severe drawbacks. The surface amplification approach works but it is impossible to make several FOCS, either individually or by a batch process, which are similar. Imbedding the chemistry in the membrane results in such disadvantages as slower response times, "leaking" of the reagents and difficulty in making uniform systems. Porous glass fibers, especially the newer ones with the larger pores (&gt;300 .mu.m), appeared to be a big breakthrough, but the large buffering capacity of the glass is a major problem as is its ability to trap unwanted molecules. The use of fluorophores, in the past, has been considered a drawback because of bleaching problems. Finally reservoir cells have been very difficult to use and to make uniformly, and it is also burdensome to obtain reliable data.
Evanescent wave FOCS and side coated FOCS, particularly the multilayered FOCS where the reacting chemistry is sandwiched between the core and clad as described in copending U.S. patent application Ser. No. 046,986, now U.S. Pat. No. 4,846,548, are alternative FOCS designs where the sensing material can be placed on the sides of the fiber.
The need for high resolution, long active lifetimes and good reproducibility between sensors for a particular species indicates the desirability of an improved reservoir sensor. The sensing agents are in liquid form and kept in the sensor by the gas permeable membrane. All of the key elements of the sensor should be very accurately controlled. Ideally, reagent solution can be made with a very high degree of repeatability, the active volume can be precisely controlled, the field-of-view of the fiber is accurately known and the size of the membrane can be held to good tolerance. Thus a design is needed which can incorporate all these features and advantages. The big drawback of the reservoir FOCS is size. It is about 1 cm in diameter compared to other FOCS which are 400 to 600 .mu.m.
Thus while reservoir FOCS would be particularly desirable for many applications, including sampling in an aqueous environment, present sensors are inadequate.
U.S. Pat. Nos. 4,737,343 to Hirshfeld and 4,666,672 to Miller et al. show typical reservoir FOCS formed by attaching a capillary tube coaxially to the end of an optical fiber using a gas bubble or membrane to close the tube. This structure is difficult to assemble accurately, difficult to control and use and impossible to reproduce uniformly. Another type of FOCS, while not actually a reservoir type, is shown by U.S. Pat. No. 4,476,870 to Peterson et al. wherein a porous polymer jacket or envelope is placed at the end of a pair of fibers and encloses a fluorescent dye on a solid (particulate) support.
The ability to detect or monitor trace amounts of chemical species in situ is of great importance and generally difficult to do. Particular applications include environmental monitoring, pollution control, public health and safety, and industrial monitoring. Groundwater and seawater monitoring are very important. The improved reservoir FOCS would be particularly desireable for many of these applications.
Trichloroethylene (TCE) heads the U.S. Environmental Protection Agency (EPA) list of hazardous (toxic, carcinogenic, etc.) compounds and the organic chlorides, as a group dominate the top ten (10) most frequently found dangerous compounds. Furthermore, total organic carbon is an indicator of the total amount of organic species present. Thus the ability to monitor TCE vapor, total organic chloride (TOCl) and total organic carbon (TOC) are very significant.
TCE is of particular concern because it forms the carcinogen vinyl chloride in water. Moreover, it is estimated that about 23 million people in the United States are exposed each year to TCE levels ranging from 500 ppb to 5 ppm even though &lt;5 ppb is considered the safe limit.
Drinking water standards require that these compounds have a concentration of &lt;5 ppb if the water is to be considered safe to drink. To assure that the EPA standards are met dictates that drinking water be monitored on a routine basis. To accomplish this task requires that a device be developed that is both inexpensive to purchase and operate and that can give reliable results in the hands of a moderately trained field technician.
The importance of safe and plentiful ground water supplies cannot be overstated. Yet domestic water quality is being threatened in many areas by the intrusion of toxic contaminants into the soil and the ground water from agricultural runoff of pesticides and herbicides; industrial discharge into lakes and rivers; and seepage from solid waste sites (landfills, storage lagoons, and waste piles).
Unlike surface contaminants, which are quickly diluted, chemicals in the soil and ground water often remain highly concentrated both underground and in the water which flows from the faucet. The potential magnitude of the problem is enormous. The EPA has identified thousands of industrial sites containing potentially hazardous wastes which have no safeguards to prevent seepage and there are over 275,000 Subtitle D municipal and industrial sites which may contain dangerous materials. It is, therefore, essential that an economical, practical water monitoring system be in place as soon as possible.
In order to provide adequate protection of water sources, methods of detecting low concentration of toxic contaminants are urgently needed. The public health, as well as the public's confidence in domestic water supplies, requires an early warning system so that prompt action may be taken to track down the sources of the contamination and to take appropriate steps to protect the public. In order to assure soil and water quality first requires that the contaminants be identified. Presently, sophisticated state-of-the-art equipment and methodologies have been used for diagnostic investigation. Wells sometimes must be drilled for proper access to the vadose zone and ground water. Typically gas chromatography, mass and atomic (absorption and emission) spectroscopy have been used in conjunction with special pumps and samplers to collect the soil and water to be analyzed. Unfortunately, present technologies are not highly suitable for continuous and widespread monitoring of groundwater contamination. Problems include the contamination of samples by well construction materials, degradation of sample integrity by most sampling techniques which could result in questionable data and make enforcement difficult, the high capital investment in complex equipment, and the need for highly skilled technicians. An improved reservoir FOCS which is accurate and uniform over a large number of sensors could be the basis of a monitor system.
Another area of application is the continuous sensitive monitoring of certain parameters, particularly oxygen and carbon dioxide in seawater. Dissolved oxygen (DO) is necessary to support all life in the marine environment and is therefore the most important water quality parameter. DO concentration is controlled by a process known as oxygen demand. Oxygen demand materials require oxygen for degradation which results in a depletion of ambient DO levels, thereby depriving marine organisms. Man made wastes, such as sewage sludge and other forms of organic wastes, are examples of oxygen demand materials. These wastes are also known to cause increased nutrient loading resulting in excessive marine plant growth. It is therefore essential to control the amount and type of waste materials dumped into marine waters and to routinely measure and monitor dissolved oxygen to ensure that adequate levels are available to support marine life.
Conditions of low dissolved oxygen often occur in highly populated estuarine and coastal areas. In the summer of 1976, a major oxygen depletion event occurred in the waters off the north eastern coastal United States. Appropriate FOCS technology would be useful for measuring and monitoring changes in oxygen and other biologically important compounds in seawater.
The growing concern over increasing levels of carbon dioxide and other trace gases on a global scale has warranted the need for improved methods for oceanic and atmospheric gas analysis and monitoring. According to recent predictions, "greenhouse" gases in the atmosphere are on a continuous rise and will result in a global warming as much as 2.degree. to 3.degree. C. in the next 50 years.
The ocean flux process permits carbon dioxide to escape from the ocean and depleted oxygen to be replenished. An in-situ measurement capability for dissolved carbon dioxide in the ocean is an important first step toward understanding the ocean flux process. This capability is needed by researchers engaged in ocean and global climate studies. FOCS have the potential to satisfy this need, which was previously unattainable with existing instrumentation.
The need for in situ monitoring of key chemical components in seawater is becoming increasingly more apparent. These components are currently measured by collecting water at the site using specially designed water samplers operated from a manned submersible or from remotely operated vehicles. These samplers are made of titanium, are expensive, and time consuming to clean and maintain. The water samples are then brought to the surface and analyzed by conventional laboratory techniques to determine the results.
There are presently a very limited number of FOCS systems available for commercial use in aqueous systems and virtually none available for use in seawater. Present day methods for the direct measurement of specific chemical parameters in seawater include deployed seawater probes for dissolved oxygen and pH that are based on electrochemistry. These Clark-type probes work either on the galvanic or polarographic principle and have a number of problems including slow response time, reproducibility, chemical and biological fouling of protective membranes, and other effects of high salinity or sensor performance and sensor lifetime. Fiber optic chemical sensors can overcome these problems and will offer many advantages, including batch fabrication at reasonable cost, expendability, small size, light weight and freedom from electromagnetic interference. In FOCS, signals are transmitted optically rather than electrically, which becomes advantageous when handling and deploying long lengths of sea cable in electrically noisy shipboard environments. A reservoir FOCS adapted to these types of measurements will make a groundwater or seawater monitoring system feasible.